The use of springs to partially support or "balance" the weight of a window sash is almost universal in the building trade. Arrangements of sash weights, pulleys and cords which were used in the past have practically gone out of existence. A spring balance cannot exactly match the weight of a sash over its travel, however, because the spring force varies with the spring extension. A spring balance holding device for a sash must have some means for adjustment of the net force applied by the spring, or the sash will not stay in position at all points of its travel. Many spring-actuated sash holding devices have been designed and are in commercial use. For example, the "Automatic Friction Sash Holder" of U.S. Pat. No. 4,571,887, issued to Garry P. Haltof, uses the vertical motion of two opposed wedges to develop a sideways motion and a frictional force against the jamb liner proportional to the force of the spring acting against the weight of the sash. The frictional force produced by the wedges resists vertical motion of the sash, and if the difference between the upward force of the spring and the weight of the sash is less than the frictional force, the sash will remain stationary. To move the sash up or down, the operator of the window only needs to apply a force slightly greater than the residual of friction: the sash will move easily but stay in place when the external force is removed.
There are problems with the Haltof sash holding device and others like it, however, the jamb liner and sash holder surface areas which develop frictional force are small, and the initial proportionality between vertical force and frictional force degrades rapidly with wear and distortion of the parts. It is extremely difficult to design a sash holder which has a proportional frictional force great enough to hold the sash, a low wear rate, and at the same time is free from "lockup" or excessive stick-slip during sash motion.
By changing the manner in which frictional force is devloped from the vertical force, I have improved the sash holder and at the same time simplified it. The improvements are given in detail in the following specification and accompanying drawings, which describe and illustrate a preferred embodiment of the Spring Force Compensator.